What is the diagnostic approach for gout?

How to Diagnose Gout

The definitive diagnosis of gout requires identification of monosodium urate (MSU) crystals in synovial fluid or tophus aspirate using polarized light microscopy, which remains the gold standard with sensitivity and specificity approaching 100%. 1, 2

When to Perform Joint Aspiration

Synovial fluid analysis should be performed when clinical judgment indicates diagnostic testing is necessary, particularly when septic arthritis cannot be excluded or when the clinical presentation is atypical. 1, 2

Indications for aspiration include:

• First presentation of acute monoarthritis where diagnosis is uncertain 1, 2
• Any suspicion of concurrent septic arthritis (fever, systemic symptoms, overlying skin wound) 1, 2
• Polyarticular presentation or atypical joint involvement 2
• Failure to respond to empiric gout therapy 2

Critical technical requirements:

• Polarizing microscope with trained operator available 1
• MSU crystals appear needle-shaped with strong negative birefringence 1, 2, 3
• Always perform Gram stain and culture even when MSU crystals are identified, as gout and septic arthritis can coexist 1, 2

Clinical Diagnosis When Aspiration Is Not Feasible

When joint aspiration cannot be performed or is declined, clinical diagnosis using validated algorithms achieves >80% sensitivity and specificity for gout. 1, 2

The ACR/EULAR 2015 classification criteria (score ≥8 diagnostic):

• Sensitivity 92% and specificity 89% with imaging 2, 4
• Sensitivity 85% and specificity 78% without imaging 2

High-value clinical features that strongly suggest gout:

• Podagra (first metatarsophalangeal joint involvement): sensitivity 96%, specificity 97% 1
• Maximum pain intensity reached within 6-12 hours of onset 2, 5, 6
• Erythema overlying the affected joint: sensitivity 92%, specificity 62% 1
• History of previous similar attacks with complete resolution 1, 2
• Presence of visible tophi: sensitivity 30%, specificity 99% 1
• Male sex, cardiovascular comorbidities 2, 4

Common pitfall to avoid:

Serum uric acid levels do not confirm or exclude gout—many people with hyperuricemia never develop gout, and during acute attacks serum levels may be normal in up to 40% of patients. 1, 5

Advanced Imaging When Diagnosis Remains Uncertain

Ultrasound (first-line imaging modality):

• "Double contour sign" on cartilage surfaces: sensitivity 74%, specificity 88% 2, 4
• Detection of tophi: sensitivity 65%, specificity 80% 2
• Better performance in disease duration >2 years (63% sensitivity vs 50% in early disease) 2
• Always include the first metatarsophalangeal joint in ultrasound examination regardless of symptoms 2

Dual-Energy CT (DECT):

• Sensitivity 85-100%, specificity 83-92% for detecting MSU crystal deposits 2, 4
• Most accurate in patients with disease duration >2 years 2
• Less sensitive in early disease (<2 years) 2
• When characteristic MSU deposits are identified on DECT or ultrasound, synovial fluid analysis is not needed to confirm diagnosis 2

Plain radiography:

• Not useful for diagnosing acute or early gout 1
• May show typical features only in chronic gout (erosions with overhanging edges, asymmetric swelling, subcortical cysts) 1
• Useful for differential diagnosis and assessing structural damage 2, 4

Diagnostic Algorithm

1. If joint can be aspirated safely and resources available → perform synovial fluid analysis for MSU crystals plus Gram stain/culture 1, 2

2. If aspiration not feasible but presentation is classic (podagra, rapid onset, hyperuricemia, previous similar attacks, no infection concern) → clinical diagnosis acceptable 1, 2

3. If diagnosis uncertain after clinical assessment → ultrasound of symptomatic joint plus first MTP joint 2

4. If ultrasound negative or unavailable and disease duration >2 years → consider DECT 2

5. If all testing inconclusive → refer to rheumatology for expert evaluation 1

Essential Risk Factor and Comorbidity Assessment

All patients with suspected or confirmed gout require systematic evaluation for associated conditions: 1, 2

• Metabolic syndrome components (obesity, hypertension, hyperglycemia, hyperlipidemia) 1, 2
• Chronic kidney disease 2, 6
• Medications increasing uric acid (diuretics, low-dose aspirin, cyclosporine, tacrolimus) 2, 4
• Dietary factors (alcohol especially beer, high-fructose corn syrup, purine-rich foods) 2, 7
• Cardiovascular disease 2, 4

Special Considerations

Intercritical periods (between attacks):

• MSU crystals can be identified from asymptomatic joints during symptom-free intervals, allowing definitive diagnosis 1
• Aspiration sensitivity 70% during intercritical periods 1

Renal uric acid excretion testing:

• Indicated in patients with family history of young-onset gout, onset <25 years, or renal calculi 1